FIG. 1 schematically shows a typical onshore hydrocarbon well location and surface equipments SE above a hydrocarbon geological formation GF after a well-bore WB drilling operation has been carried out, after a casing string CS has been run and after cementing operations have been carried out for sealing the annulus CA (i.e. the space between the well-bore WB and the casing string CS).
Typically, the casing string CS comprises a plurality of casing joints CJ, two casing joints being coupled together by a casing collar CC. The casing string function is to stabilize the well-bore.
The casing joint CJ is a length of steel pipe, generally around 13 m or 40 ft long with an externally threaded (male thread form) connection at each end. Casing joints are assembled to form a casing string of the correct length and specification for the well-bore in which it is installed.
The casing collar CC is an internally threaded (female thread form) short length of pipe used to connect two casing joints. The resulting connection must provide adequate mechanical strength to enable the casing string to be run and cemented in place. The casing collar must also provide sufficient hydraulic isolation under the design conditions determined by internal and external pressure conditions and fluid characteristics.
The casing may be made of plain carbon steel, stainless steel or other material in order to withstand a variety of forces, such as collapse, burst, and tensile failure, as well as chemically aggressive fluid. Nevertheless, in harsh environment, the casing may be subject to corrosion that may affect its functionality.
At this stage, well logging operation may be carried out. The well logging operation serves to measure various parameters of the hydrocarbon well geological formation (e.g. resistivity, porosity, etc. . . . at different depths) and in the well-bore (e.g. temperature, pressure, fluid type, fluid flowrate, etc. . . . at different depths). Such measurements are performed by a logging tool TL. Generally, a logging tool comprises at least one sensor (e.g. resistivity sonde, mechanical sonde, gamma ray neutron sonde, accelerometer, pressure sensor, temperature sensor, etc. . . . ) and measures at least one parameter. It may include a plurality of same or different sensors sensitive to one or more parameters. The logging tool is moved up and down in the borehole for gathering data about the various parameters by means of a cable LN. The cable may be a mechanical cable, an electrical cable or an electro-optical cable comprising a fiber line protected against potential harsh environment existing in the well-bore. The mechanical, electrical, or electro-optical cable transmits electrical signals or optical signals from the logging tool to the surface unit.
The logging tool may be deployed inside the well-bore by an adapted surface equipment SE that may include a vehicle SU and an adapted deploying system, e.g. a drilling rig DR or the like. Data related to the hydrocarbon geological formation GF or to the well-bore WB gathered by the logging tool TL may be transmitted in real-time to the surface, for example to the vehicle fitted with an appropriate data collection and analysis computer and software.
In particular, a logging tool TL may comprise an inspection tool.
FIG. 1 also schematically shows a magnified view of a portion of the cased well-bore, wherein the tool TL comprises an inspection tool. The inspection tool can detect the position, shape and dimension of a corrosion zone CR affecting a casing joint CJ′. The tool TL provides the measurements to the surface equipment through the connection line LN. By correlating this detection with depth measurements made by the tool TL, it is possible to run an appropriate tool down-hole for providing an appropriate remedial treatment (e.g. chemical treatment, patch, casing replacement or the like) for consolidating the corroded casing joint CJ′.
The inspection tool may be a mechanical caliper, an ultrasonic tool or an electromagnetic tool.
The mechanical caliper comprises a plurality of fingers for sensing the inner geometry of the casing. The mechanical calipers cannot be used for the determination of casing thickness, cannot distinguish non-metallic deposits on the casing from the metallic casing itself, and may initiate corrosion by scratching the casing surface.
The ultrasonic tool measures the time-of-flight of a sound-pulse between emission by the tool, reflections at the inner and outer surfaces of the casing and reception by the sound-wave detectors in the tool. They may measure the inside diameter as well as the thickness of the casing. The ultrasonic tool cannot be used when the casing transports a fluid mixture comprising a certain quantity of gas relatively to liquid, and provides deteriorated reflected signals in the presence of surface roughness which typically arise in corroded casing.
The electromagnetic tool is a non-destructive inspection tool. It may be based on either the flux-leakage principle, or the eddy-current principle or a combination thereof. The electromagnetic tool is insensitive to non-conductive deposits and can operate irrespective of the nature of the fluid mixture flowing into the casing.
The electromagnetic tool based on flux-leakage principle is typically used for the detection of localized damages in a ferromagnetic casing. This tool commonly subjects the casing to a strong static magnetic field. Due to the ferromagnetic nature of the casing, the magnetic return flux is mainly confined to the inside of the metal casing. In the presence of discontinuities in the metal, such as pits and holes caused by corrosion, the magnetic flux “leaks” out of the metal casing and may be detected with appropriate magnetic sensors such as coils, Hall probes, or magneto-resistive sensors. To allow for sufficiently sensitive and quantitative measurements, the flux-leakage based tool requires strong magnetic fields and good flux coupling into the inspected casing. This implies close proximity of the magnetic field source to the inner casing surface. The combinations of casings and tubings used in hydrocarbon wells often exhibit varying casing diameters that render the requirements of sufficiently strong and well-coupled magnetic flux difficult to maintain. Furthermore, magnetic flux-leakage tools are not suited for measurements of gradual thinning of the pipes.
The electromagnetic tool based on eddy-current principle is well-suited for the measurement of both, inside diameter and wall thickness of ferromagnetic metallic casing. This tool excites an alternating current in a coil transmitter adapted for inducing eddy-currents in the surrounding conductive casing and measures the induced voltage in a separate receiver coil. Alternatively, the tool may measure the impedance of the transmitter coil. The transimpedance is affected by the casing magnetic permeability (μ), the electrical conductivity (σ) and the inner diameter of the nearby casing. In the case of electromagnetic tool where sufficiently low frequencies and large coil separations are used, measurement of the transimpedance is used to extract the ratio of wall thickness d of the casing to skin depth δ, the so-called electromagnetic thickness d/δ.
It is known that the results for inner diameter and wall thickness have to be corrected particularly for variations in magnetic permeability which can be significant, for instance due to mechanical stress or heat exposure. Document U.S. Pat. No. 4,546,314 proposes a first order correction method. However, in practice, it has been found that this first-order correction is commonly insufficient in order to achieve satisfactory measurements accuracy.